The present invention relates to electronic pressure sensing devices and in particular to digitizer pads capable of locating the leading and trailing edges of a pressure contact area.
Digitizer pads and their function are well known. Examples of prior art digitizer pads are illustrated in Eventoff U.S. Pat. No. 4,739,299, issued Apr. 19, 1988 and Margolin U.S. Pat. No. 4,455,450, issued June 19, 1984. Digitizer pads of the above construction have generally had opposing conductor pads with solid or interleaved conductor traces to enable the X-Y or XYZ coordinates of the point of force application against the digitizer pad to be identified. In general, such digitizer pads have contemplated the use of a generally pointed instrument to apply force against the pad so that the pressure point would be very localized. However, in many applications the point of contact extends over an larger area in which many adjacent circuit traces are pressed into contacting relationship which results in ambiguities since there are multiple points of pressure application. In such a case; the measurement of the position of the force usually measures the position of the barycentric average of the force, that is EQU &lt;x&gt;=.intg.x F(x) dx/.intg.F(x) dx
where &lt;x&gt; is the measured force position, F(x) is the distribution of force with position, and the integrals extend along the entire length of the position sensor. Naturally, this can be a measurement along a single coordinate axis, or along two perpendicular axes.
In some cases, however, it is desirable to measure the position of the leading or trailing edge of an object, rather than its barycentric position. This is useful for determining the size of an object, as well as a providing a dynamic means of measuring the object's position or motion.
The present invention addresses this problem by providing a digitizer pad which is capable of detecting both the leading and trailing edges bounding the area of pressure application on the digitizer pad in the spacial dimension defined by the orientation of the interleaved circuit traces so as to identify the bounds of the area of pressure application in that dimension. The digitizer pad apparatus in accordance with the invention may be made using one or more digitizer ply apparatus each with the interleaved circuit traces of each ply positioned in a predefined angular orientation relative to the traces of other plys. In one desired embodiment, the circuit traces of two digitizer ply structures are oriented orthogonal to one another in a plane. The resultant digitizer pad apparatus is an X-Y digitizer pad capable of locating the bounds of the areas over which the pressure is applied in both the X and Y dimensions. The amount of pressure applied can also be determined to define a "Z" dimension perpendicular to the plane of the X-Y plane in which the two digitizer ply structures are oriented, by utilizing a force sensing resistor ply such as that described in Eventoff, U.S. Pat. No. 4,739,299 issued Apr. 19, 1988 and Eventoff, U.S. Pat. No. 4,315,238, issued Feb. 9, 1982.